How to break to console on Mint Linux VM

This morning I updated my Mint Mate VM and rebooted as directed. The system
came up to the login screen and I entered the password. The screen expanded
to its normal size, but the login screen just appeared again. Any entry of
the password led me nowhere. I tried to boot headless, but could find no
way to get to a login prompt.

The system just returned:
ptavv> VBoxManage startvm Mint --type headless
Waiting for VM "Mint" to power on...
VM "Mint" has been successfully started.
ptavv>

I am not a Linux expert and I'm mostly clueless about admin since systemd,
and I don't make much use of this VM, but it is handy now and then.

Can anyone provide a clue on how to try to rescue the system? Clearly,
something is causing my session to exit on startup, but I have no idea what
and, without command line access, I'm unlikely to find it
-- 
Kevin Oberman, Part time kid herder and retired Network Engineer
E-mail: rkoberman@gmail.com
PGP Fingerprint: D03FB98AFA78E3B78C1694B318AB39EF1B055683